1. Field of the Invention
The present invention relates to a device for machining the ends of tubes and, more particularly, to the cutting of threaded connections in the tubes, wherein the cutting tools are displaceably and adjustably arranged in a tool head radially to the tube. The tool head rotates coaxially around the firmly clamped tube end and the tools are clamped in tool holders such as slide bars. Adjustment of the tools with respect to the tube is controlled by push rods arranged in an axially parallel relationship with the tube and distributed uniformly about its circumference. The end of each push rod remote from the end of the tube is connected to a common frame which is displaceable coaxially with the axis of the tool head. The opposite end of each push rod is functionally attached to a respective transmission device by which the axially parallel infeed movement of the push rods can be converted into synchronous radial displacement of the slide bars for adjusting the cutting tools.
2. Description of the Prior Art
Thread-cutting machines which clamp a tube and include rotating tools for machining the ends of the tube, particularly cutting thread connections on, for instance, tubing and easing tubes are well-known. The tube thread is produced, as a rule, by a single pass of the tools over the end of the tube, the movements of the tools being controlled by a computer numerical control. The corresponding machines must be designed so that particularly narrow thread tolerances can be obtained. Of particular importance in these machines is the tool head in which the tools are adjustably guided against the tube. The tube is clamped in a well-defined position with respect to the machine and, after the exact position of the end of the tube has been determined, the infeed of the cutting tool is switched on. The machining tools are displaceably arranged on the tool head and uniformly distributed around its circumference. Any number of tools may be used, although in most cases six tools are provided, three of which are pre-turning tools for the machining of the cone and the other three are combined finish-turning and thread chasing tools. The tools are arranged concentrically within the rotating tool head. In each case, three tools which are 120.degree. apart are moved in one direction and the other tools are moved in the other direction. Very high centrifugal forces act on the moving masses due to the high speeds of rotation of the tool head, e.g. 1000 rpm. These centrifugal forces counteract the infeed forces of the tools. The tools should be accurately positioned to keep the forces as small as possible.
U.S. Pat. No. 4,289,430 describes a device for the machining of a tube using only two tools. The two tools may be adjusted by push rods which are axially displaceable in parallel relation to the axis of the tool head and the workpiece under the control of a common frame with the help of a transmission device. Bevels on the tool holders serve as transmission devices for converting the longitudinal forces into transverse forces.
A disadvantage of these known devices for the machining of tube ends are that large frictional forces act on the transmission devices which are exposed to the full centrifugal forces from the rotating movement of the tool head on their bevels and oblique planes as well as on the guide paths themselves. This detrimentally affects the accuracy of the positioning of the tools and thus the manufacturing tolerances of the tube ends. Furthermore, the guides are subject to extensive wear and the device is also very costly.